The present invention relates to a method for producing colored and exactly spherical fine-grains of polymer, in which pigment is homogeneously dispersed in the exactly spherical fine-grains of polymer and, thus, loss of color is prevented.
In the past, fine-grains of cross-linked resin, which are colored with water-soluble coloring matters, have become known (laid-open Patent Publication No. 288632/1987). Moreover, a method for producing colored grains of resin, which comprises the steps of mixing a dye or pigment with ethylenic unsaturated monomers, and suspension-polymerizing the obtained mixture in water medium under the presence of suspension stabilizers, is well known. (Patent Publication No. 10231/1961).
However, the fine-grains of cross-linked resin designated in said laid-open Patent Publication, which are colored with water soluble coloring matters, are inadequate for solvent resistance and weatherability, although the loss of color when the coloring matters are dropped out from the fine-grains of resin is limited. On the other hand, colored grains of resin which are obtained by merely blending pigments are comparatively good in solvent resistance and weatherability when mixed with paint. However, in the case of exactly spherical fine-grains of polymer using inorganic pigments, the loss of color is significant because the surfaces of the inorganic pigments show hydrophilic properties.
Therefore, when the untreated inorganic pigments are mixed with the ethylenic unsaturated monomers, and subsequently the suspension polymerization is carried out in water medium, the inorganic pigments are transferred into the water medium, and partially fixed on the surfaces of the exactly spherical fine-grains of polymer. The inorganic pigments fixed on the fine-grains of polymer are then dropped out from the polymer grains and, as a result, loss of color occurs and colored exactly spherical fine-grains of polymer containing smaller amounts of inorganic pigments therein are obtained.
The present inventors have made various studies to eliminate the aforementioned faults and consequently achieved this invention by finding that the colored and exactly spherical fine-grains of polymer, which do not exhibit a loss of color and are uniformly formed in the grain size of 5 .mu.m to 100 .mu.m, can be obtained by carrying out the suspension polymerization with the use of inorganic pigments which are surface-treated with a silane-coupling agent.